Color metadata for a downlink data channel

ABSTRACT

The present invention provides a method and system for determining and providing color metadata for a downlink data channel. In accordance with the present invention color metadata is determined such that gamut mapping is defined by a transmitted source gamut which enables adapted mapping that preserves saturation and contrast levels of the source for a downlink data channel. The metadata is then provided to a downlink data channel for color management.

This application is a continuation of prior U.S. application Ser. No.12/224,538, filed on Aug. 28, 2008.

FIELD OF THE INVENTION

The present invention generally relates to color management and, moreparticularly, to determining and providing color metadata for a downlinkdata channel for enabling improved gamut mapping and color management.

BACKGROUND OF THE INVENTION

In color management color spaces are discriminated as device dependent(DD) and device independent (DI) color spaces. Device dependent colorspaces are generally the RGB and xvYCC color spaces (proposed by Sony™in 2005 for HDMI 1.3). Device dependent color signals result in areproduced color that depends on the display device. Device independentcolor spaces are typically CIE XYZ and CIE L*a*b*. Device independentcolor signals correspond directly to the reproduced color. The linkbetween DD and DI color signals for a given display device or for agiven pictorial content is defined by a profile.

When connecting a HDMI source to a HDMI sink, source and sink aredescribed by profiles. One part of a profile is the color gamut. Thecolor gamut describes all colors that are reproducible by a given HDMIsink display or that are present in a given content. The color gamut canbe described by a Gamut Boundary Description (GBD). When a given imagecontent has a gamut larger or different from the gamut of the HDMI sink,the colors lying outside the aimed gamut have to be clipped or movedaccordingly. This procedure is called Gamut Mapping (GM). The gamut ofthe content is circumscribed by the gamut of the HDMI source.

SUMMARY OF THE INVENTION

The present invention advantageously provides a method and system fordetermining and providing color metadata for a downlink data channel. Inaccordance with the present invention color metadata is determined suchthat gamut mapping is defined by a transmitted source gamut whichenables adapted mapping that preserves saturation and contrast levels ofthe source for a downlink data channel. The metadata is then provided toa downlink data channel for color management.

In one embodiment of the present invention, a method for determiningcolor metadata for a downlink data channel for preserving a color gamutof content of a source includes defining a color space gamut using thecolor gamut of said source, and performing gamut mapping using thedefined color space gamut.

In an alternate embodiment of the present invention the method includesdefining the color space gamut in xvYCC space.

In various embodiments of the present invention, a color spacedefinition comprises a gamut boundary description and an indexed facetset is implemented to define the color space gamut. Alternatively, aregular sampling scheme can be used to define the color space gamut.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 depicts a high level block diagram of a gamut mapping system inwhich an embodiment of the present invention can be applied;

FIG. 2A depicts a prior art example of gamut mapping in which an entirecolor space gamut (the large rectangle) is able to be mapped onto thegamut of an HOMI sink gamut (the triangle); and

FIG. 2B depicts a proposed gamut mapping approach in accordance with anembodiment of the present invention.

It should be understood that the drawings are for purposes ofillustrating the concepts of the invention and are not necessarily theonly possible configuration for illustrating the invention. Tofacilitate understanding, identical reference numerals have been used,where possible, to designate identical elements that are common to thefigures.

DETAILED DESCRIPTION OF THE INVENTION

The present invention advantageously provides a method and system fordetermining and providing color metadata for a downlink data channel.Although the present invention will be described primarily within thecontext of a HDMI TMDS downlink data channel, the specific embodimentsof the present invention should not be treated as limiting the scope ofthe invention. It will be appreciated by those skilled in the art andinformed by the teachings of the present invention that the concepts ofthe present invention can be advantageously applied for providing colormetadata for substantially any downlink data channel for enablinggreater gamut mapping.

Furthermore, although the present invention will be described primarilywithin the context of specific colorimetries and color space indicators,the specific embodiments of the present invention should not be treatedas limiting the scope of the invention. It will be appreciated by thoseskilled in the art and informed by the teachings of the presentinvention that the concepts of the present invention can beadvantageously applied using substantially any colorimetries and colorspace indicators, whether known or unknown.

This present invention is described primarily within the context ofcolor metadata for an HDMI TMDS downlink data channel, however, thecolor management principle will not be thoroughly discussed or definedherein, since the colorimetry of a content signal is already defined bythe color space used.

A motivation for the color metadata of the present invention is that inusing current gamut mapping methods, colors are out of gamut of the HDMIsink that need to be gamut mapped. However, the downlink metadata cannot cover the natural properties of the HDMI sink. In addition, thecolor metadata of the present invention can not cover the gamut mappingprocess since gamut mapping depends on the properties of the HDMI sink.Instead, the downlink color metadata of the present invention addressesthe properties of the HDMI source.

FIG. 1 depicts a high level block diagram of a gamut mapping system 100in which an embodiment of the present invention can be applied. Morespecifically, as depicted in FIG. 1, original media content 102 iscommunicated downstream with color metadata 104 in accordance with thepresent invention. The media content 102, the color metadata 104 andproperties of the display 106 are all used for gamut mapping. In thepresent invention, gamut mapping and display properties are notaddressed. That is gamut mapping is a well-known topic in the art asevidenced by the scientific literature Morovic J. and Luo M. R., TheFundamentals of Gamut Mapping: A Survey, Journal of Imaging Science andTechnology, 45/3:283-290, 2001.

As previously stated, the sink profile is already known by the colorspace used and needs not be included in the metadata of the presentinvention. However, the gamut defined by the color space can be verylarge, specifically in case of wide gamut color spaces. When the gamutof the color space in use is much larger than the gamut of the imagecontent, gamut mapping may be less efficient.

FIG. 2A depicts a prior art example of gamut mapping in which an entirecolor space gamut (the large rectangle) is able to be mapped onto thegamut of an HDMI sink gamut (the triangle). Such mapping compressesimage content with a gamut (the ellipse) smaller than the color spacegamut onto an image having a gamut (the dotted ellipse) smaller than thegamut of the HDMI sink. However, when using such gamut mapping, theimage loses saturation and contrast and the capacities of the sink arenot exploited.

FIG. 2B depicts a proposed gamut mapping approach in accordance with anembodiment of the present invention in which the description of thegamut of the HDMI sink (the “source gamut” rectangle) is used toaccomplish the gamut mapping. In the gamut mapping of FIG. 2B, themapping is less strong and the gamut of the media content is betterpreserved. That is, gamut mapping defined by transmitted HDMI sourcegamut in accordance with the present invention enables adapted mappingthat preserves saturation and contrast and exploits the capacities ofthe sink.

In one embodiment of the present invention, the requirements for thesink gamut boundary description for the color metadata of the presentinvention include 1) High precision; 2) Concise representation and 3)High processing speed and low processing complexity.

For example, the first requirement is high precision. This requirementdepends on how a source gamut boundary description (GBD) is calculated(out of scope of HDMI) and on color space precision (within scope ofHDMI). For a source GBD calculation, two cases can be distinguished asfollows:

-   -   1. Source GBD is voluntarily larger than source gamut color        reproduction will be slightly deteriorated, but GBD precision is        less important.    -   2. Source GBD is close to source gamut best color reproduction,        but GBD precision needs to be maximal.

Because the calculation of the sink GBD is not in the scope of HDMI, GBDprecision should be high. The precision of GBD needs to be related tothe precision of the color space of the signal. Since gamut mapping isapplied to the signal, GBD precision can always be less or equal thancolor space precision. GBD precision can be modulated by the choice of acolor space independent from signal color space.

The second requirement for the sink gamut boundary description for thecolor metadata of the present invention is concise representation whichis inversely correlated to the first requirement. That is, conciserepresentation is in general a data compression problem. A concise GBDdepends on the shape of a source gamut which is out of scope of HDMI.Parametric representations cannot be used when sources with an unknownnumber of primaries, with unknown tone reproduction and of unknown type(additive, subtractive) are included in the samples. Gamuts may not evenbe convex. If the shape of the source gamut is not known, a sourceadapted representation can not be used. Instead, general assumptionssuch as smoothness and regularity can be applied. When the source gamutis smooth, a sampling scheme is efficient. When the source gamut isregular even regular sampling can be used.

The third requirement for the sink gamut boundary description for thecolor metadata of the present invention is high speed and low complexityof GBD processing in the HDMI sink. When using a sampling scheme for asource GBD, the easiest processing would be on samples that are in asense regular. If the second requirement demands a more conciserepresentation based on irregular sampling, the sampling order or thesamples structure can be added to the samples in order to speed upprocessing. In accordance with one embodiment of the present invention,a possible representation is an indexed facet set, since today'shardware supports geometrical operations such as line to surfaceintersections.

The inventors depict herein a generic example of the concepts forproviding metadata for a downlink data channel in accordance with thepresent invention followed by more specific examples.

That is in the various embodiments of the present invention describedbelow, the color downlink metadata uses one of the followingcolorimetries:

-   -   ITU-R BT.601;    -   ITU-R BT.709;    -   CIE XYZ.        Furthermore, the color downlink metadata of the present        invention describes the HDMI source gamut using one of the four        color spaces indicated by a color space indicator:    -   ITU-R BT.709;    -   IEC 61966-2-4—SD    -   IEC 61966-2-4—HD    -   DCI specification V5.1.

Table 1 below depicts a generic example of a configuration table fordefining the color space of the color downlink metadata of the presentinvention:

TABLE 1 Name of field Symbol Size Description GBD_COLOR_SPACE 3 bitColor space of GBDThe gamut boundary description (GBD), in accordance with the presentinvention, can therefore be either Device dependent (ITU-R BT.709-4, IEC61966-2-4) or Device independent (DCI).

The color encoding (color space precision) of the present invention isdefined according to the chosen color space. As such in the embodimentof the present invention described below, to be harmonized with otherparts of HDMI 1.3, color precision can be defined as:

-   -   3×8 bit    -   3×10 bit    -   3×12 bit

Table 2 below depicts generic example of a configuration table fordefining the color precision of the color downlink metadata of thepresent invention:

TABLE 2 Name of field Symbol Size Description GBD_COLOR_PRECISION N 2bit Color precision: 8 bit, 10 bit, 12 bitIn one embodiment of the present invention, the HDMI source gamut isdescribed by a set of vertices with or without indexed facets in thechosen color space as depicted in Table 3, which follows:

TABLE 3 Name of field Symbol Size Description NUMBER_VERTICES N_(V) 16bit Number of vertices GAMUT_VERTICES typically Packed vertices <18.5 KBFACET_MODE 1 bit Switch for indexed vertices yes/no NUMBER_FACETS N_(F)16 bit Number of facets GAMUT_FACETS typically Packed facet indices<18.5 KBThe size of the GAMUT VERTICES and GAMUT FACETS fields of the GBDmetadata can be characterized according to equation one (1), whichfollows:S=3N _(V) N+3N _(F) ┌ld(N _(V))┐  (1)In equation (1) above, ┌*┐ depicts an operation that rounds to the nextupper integer. Furthermore, in equation (1) “ld” depicts the logarithmto the base of 2. In order to limit the size of the metadata block, theconstraint S<S_(max) applies to the choice of N_(V) and N_(F) whileS_(max) depends on the transport mechanism. In a typical application ofthe present invention, a plausible value for S_(max) is 40 KB.

In addition, in various embodiments of the color downlink metadata ofthe present invention, fields are included to indicate at least a “Startof metadata package” and “Validity period (e.g., time stamps, framereferences, etc)” for purposes of synchronization.

As previously stated, the inventors provide below a specific example ofa color downlink metadata in accordance with an embodiment of thepresent invention for a three primary media content. In the examplebelow, the gamut is described in xvYCC space (proposed by Sony™ in 2005for HDMI 1.3) at 8 bit color encoding. The GBD depicted in Table 4 belowconsists of black point, white point, as well as of red, green and blueprimaries. The total size of the metadata block is 144 bit as follows:

TABLE 4 Name of field Symbol Size Contents GBD_COLOR_SPACE  3 bit xvYCCspace GBD_COLOR_PRECISION N  2 bit 3 × 8 bit NUMBER_VERTICES N_(V)  16bit 5 GAMUT_VERTICES 120 bit Packed vertices FACET_MODE  1 bit Indexedvertices: no Total size 144 bit

A second example is presented below for a GBD at visibility threshold.The GBD is constructed in CIE L*a*b*. It is assumed that the gamutboundary is smooth with a maximal slope in L*a*b* space of 4%. Furtherassuming a visibility threshold characterized according to equation two(2), which follows:ΔE=√{square root over ((ΔL)²+(Δa)²+(Δb)²)}=1  (2)

Using the conditions described above, it is concluded that the distancebetween samples representing the boundary need to be less than ΔE=25.The GBD is set to a cuboid with the limits [0,116], [−200,200],[−500,500] for L*, a*, b*, respectively. In order to cover the surfaceof the cuboid by samples having a distance of ΔE=25, N_(F)=3080triangles are required and N_(V)=1540 vertices are required. SelectingXYZ space at 10 bit, each vertex requires 30 bit. Each facet requires anadditional 36 bit. With N_(V)=1540, vertices can be indexed using 12bit. As such, the total size of this metadata packet is 19.1 KB asdepicted in Table 5, which follows:

TABLE 5 Name of field Symbol Size Contents GBD_COLOR_SPACE   3 bit CIEXYZ space GBD_COLOR_PRECISION N   2 bit 3 × 10 bit NUMBER_VERTICES N_(V)  16 bit 2812 GAMUT_VERTICES  5.6 kB Packed vertices FACET_MODE   1 bitIndexed vertices: yes NUMBER_FACETS N_(F)   16 bit 5624 GAMUT_FACETS13.5 kB Packed facet indices Total size 19.1 kB

The metadata can be provided to a downlink data channel on a separatestream or alternatively as part of the original media content. Inalternate embodiments of the present invention, the determined metadataof the present invention can be provided for use by a downlink datachannel or device on a storage medium such as an optical disk (e.g.,DVD) as part of original content or as a separate stream or signal.

Having described preferred embodiments for a method and system fordetermining and providing color metadata for a downlink data channel(which are intended to be illustrative and not limiting), it is notedthat modifications and variations can be made by persons skilled in theart in light of the above teachings. It is therefore to be understoodthat changes may be made in the particular embodiments of the inventiondisclosed which are within the scope and spirit of the invention asoutlined by the appended claims. While the forgoing is directed tovarious embodiments of the present invention, other and furtherembodiments of the invention may be devised without departing from thebasic scope thereof. As such, the appropriate scope of the invention isto be determined according to the claims, which follow.

The invention claimed is:
 1. A method for gamut mapping a color gamut ofcontent of a source, comprising: accessing metadata describing a colorgamut boundary of content of a source; communicating the metadata to asink, via a data channel, for use in the gamut mapping; and performingthe gamut mapping based on the metadata for display of the content atthe sink, wherein the color gamut is described using a set of verticesin a chosen color space, and further wherein the metadata describing thecolor gamut boundary of the source comprises a set of data including:color space indicator that is indicative of the color space associatedwith the gamut boundary description; color precision indicator that isindicative of a color space precision associated with the gamut boundarydescription; number of vertices indicator indicative of number ofvertices associated with the gamut boundary description; gamut verticesdata associated with the gamut boundary description; and facet modeindicator that is indicative of whether facet data associated with theset of vertices is included with the communicated metadata, wherein ifthe facet mode indicator indicates facet data is included, facet dataassociated with the set of vertices is included in the set of data,else, facet data is not included in the set of data.
 2. The method ofclaim 1, wherein the color space indicator indicates that the colorspace associated with the gamut boundary description is a color spacecomprising one of ITU-R BT.709, IEC 61966-2-4-SD, and IEC 61966-2-4-HD.3. The method of claim 2, wherein the color precision indicatorindicates that the color precision is one of 8 bit, 10 bit and 12 bit.4. The method of claim 1, wherein if the facet mode indicator indicatesthat facet data is included in the gamut boundary description, the facetdata included in the set of data includes: number of facets indicatorindicative of number of facets associated with the gamut boundarydescription; and gamut facet data associated with the gamut boundarydescription.
 5. The method for gamut mapping according to claim 4,wherein the color precision indicator indicates that the color spaceprecision is one of 8 bit, 10, and 12 bit precision.